The extremely long DNA molecule is actually made of a long string of chemical building blocks called “nucleotides.” There are four different nucleotides, which are labeled adenine (A), thymine (T), guanine (G), and cytosine (C). The human genome is made of a sequence of roughly three billion of these nucleotides, and it is about the same size as the genome of a chimpanzee or a mouse. In contrast, a fruit fly has 180 million, a yeast has 12 million, and the flowering weed “thale cress” has 100 million nucleotides of DNA in its genome.

Learn more about how to read the DNA sequence and probe the sequence for matches in the following sections:

Probe the Sequence
DNA sequences hold the instructions for building
organisms. In this online activity, you can explore a
sequence by designing and sending short DNA "probes" through
part of the human genome.